Donnan introduced this theory in 1911-1914. According to this theory, there is a fixed anion or anion charge inside the cell membrane that cannot be absorbed. To neutralize this fixed negative charge, the cation or cationic charge moves in from the outside of the cell membrane in the diffusion process. This process continues until there is an equilibrium of cations and anions inside the cell. Donnan calls this phenomenon equilibrium. In Donnan equilibrium, the ratio of positive charges inside and outside the cell is equal to the ratio of negative charges inside and outside the cell.